The invention relates to cyano-substituted biphenyl compounds and their use for modifying the dielectric anisotropy (DA) of liquid crystalline dielectrics.
Liquid crystalline dielectrics having a clearly negative DA are required for electrooptical display devices, the mode of operation of which is based on the phenomenon of dynamic scattering. Additional requirements to be met by liquid crystalline dielectrics are high chemical stability; a broad temperature range, which includes room temperature, for the nematic phase; and a viscosity as low as possible.
These requirements are only partially satisfied by conventional liquid crystalline dielectric materials of the Schiff base, azoxybenzene, benzoic acid phenyl ester, and thiobenzoic acid phenyl ester types. Schiff bases are chemically unstable, the azoxybenzenes are sensitive to light and UV irradiation, and benzoic acid and thiobenzoic acid phenyl esters are too viscous. Most of the liquid crystalline compounds of these classes of substances have a DA value close to zero.
In order to produce dielectrics having clearly negative DA values, these liquid crystalline compounds can be mixed, for example, with the liquid crystalline compounds of the 2'-cyano- (or 2'-nitro-) -4'-alkyl- (or 4'-alkoxy-)!-phenyl esters of 4- 4-alkyl- (or 4-alkoxy-) benzoyl-oxy!benzoic acid series in accordance with DOS (German Unexamined Laid-Open Application) No. 2,240,864, now U.S. Pat. No. 3,953,491, incorporated herein by reference. These additives all have a high clearing point and, with adequate solubility, can broaden the nematic range of the thus-prepared dielectrics in the desired way.
Unfortunately, however, they also impart to the mixtures high viscosity and accordingly cause undesirably long switching times in electrooptical display devices constructed therefrom.